1. Field of the Invention
The present invention relates to machining and, more particularly, to drilling holes, slots and other passageways through a workpiece by electron beam and other energy beam techniques.
2. Description of the Prior Art
The use of electron beam energy to drill one or more holes in a metallic or nonmetallic workpiece has only recently been investigated. It has been discovered from experimental electron beam drilling tests that in order to produce a hole symmetrical along its length through the workpiece, the electron beam must have a certain amount of excess energy, that is, more energy than merely required to achieve penetration through the workpiece. If this excess beam energy is not provided, a hole of nonsymmetrical cross-section, or tapered shape, or both will be produced. As a result of the requirement for excess electron beam energy, a material, or so-called backer, is required at the surface of the workpiece facing away from the beam, that is, the surface which will be last penetrated by the beam as it progresses through the workpiece. Hereafter this surface is referred to as the exit surface. The purpose of the backer material is to both absorb or dissipate the excess electron beam energy as the beam penetrates the exit surface and to generate sufficient gaseous pressure by the local action of the beam to expel the molten workpiece material from the hole being drilled. The molten material is typically expelled in the direction opposite to the propagation of the beam, that is, from the entrance of the hole at the first penetrated surface, or entrance surface, of the workpiece. When the expulsion is insufficient a burr will be formed around the entrance to the hole. This is often coupled with a "recast" layer, or a portion of melted and solidified metal within the hole.
Backers of metals, such as brass or zinc, have been used in the development of electron beam drilling. These are usable when they have higher vapor pressure than the workpiece, such as is true with steel. Metallic backers have also been used in combination with volatilizing films, which supply the gaseous medium to expel the molten metal from the drilled hole in the workpiece, as described in Konig U.S. Pat. No. 3,649,806. However, many workpieces used in high performance situations are made of materials sensitive to contamination. Here the use of metallic backers will disadvantageously lower material properties because of alloying at the hole location. This happens, for instance, when zinc backers are used in combination with high temperature and fatigue resisting nickel alloys. Premature failure will occur at the holes due to contamination. Other backer materials can similarly cause degradation by lowering properties or inducing corrosion in service.
Combinations of metal powders, and various organic matrices have also been used heretofore. The use of backers containing metal powder has the potential contamination disadvantage previously mentioned for metallic films. In addition particulate residue left in the interior of a hollow drilled component can react with the component on heating during service with adverse result. With metal powders the organic binders used heretofore have generally comprised materials such as silicon rubber and epoxies. These have been removed only by mechanical means or combustion, inasmuch as they are not conveniently solube in commercial solvents which will not also attack the workpiece.
Preformed metal and ceramic cellular structures containing volatile materials have also been used, as described in Konig, supra. However, these have the disadvantage, as do metallic sheets to a lesser degree, that they cannot be made to closely conform to irregular surfaces, and are generally unsuitable for placement in, or removal from, complexly shaped interior cavities. When close contact with the workpiece is not maintained backers function inferiorly and further, an undesirable burr may be formed at the exit surface.
Backers, as mentioned, are used in order to improve the dimensional quality of the hole being drilled. But heretofore there has been little more than general knowledge on the specific correlation between the physical characteristics of the backer material and the character of the drilled hole. Therefore, the artisan is not instructed by the prior art on selection of a backer material to overcome problems with particular hole configurations or workpiece materials. For example, perpendicular holes can be drilled in certain nickel base alloys, using prior art backer materials or structures. However, using the same materials, irregular and undesired shapes occur when drilling oblique holes in other cast nickel base superalloys. As a further example, the energy to be absorbed and the effects to be accommodated by the backer are considerably greater for deeper holes than shallow holes. Consequently some backers work well in the second instance but poorly in the first.